The present invention relates to a depression responsive switch unit which is turned on in response to the depression of a knob.
A conventional depression responsive two-step switch unit which is disclosed in Japanese Laid-Open Patent Application No. 315,682/96 (laid open Nov. 29, 1996) will be briefly described with reference to FIGS. 1, 2, 3A and 3B. A rectangular case 2 includes a surface plate 2a, in which a rectangular opening 2-1 is formed, and a key top 3 is disposed to substantially block the opening 2-1. The key top 3 includes an elongate frame-shaped sidewall 3g, on the inner surface of which a reinforcing plate 3c is fitted and is secured in position by claws 3d. A membrane sheet 6 is held in overlapping relationship with the front side of the reinforcing plate 3c, and a surface sheet 3e is disposed on the front side of the membrane sheet 6 and is adhesively bonded to the end face of the sidewall 3g of the key top 3.
A plurality of depression regions 3p are defined in an array on the surface sheet 3e, and are designated by key identification characters, which are numbers “1”, “2”, . . . , “8” in the example shown. Regions on the membrane sheet 6 which are located opposite to the depression regions 3p are each designated as a membrane switch 6s. Specifically, a pair of flexible films 6a and 6b, as may be formed by polyethylene films, are stacked together with a spacer 6c therebetween to define a switch assembly for each depression region 3p. In each switch assembly, fixed contacts 6d and 6e are formed on the film 6a while a movable contact 6f is formed on the film 6b so as to be located opposite to the contacts 6d and 6e, thus completing the membrane switch 6s. 
Rotary shafts 3f project externally from the opposite ends of the sidewall 3g of the key top 3 and are rotatably engaged with bearings 2f which are formed in the internal surface of the surface plate 2a of the case.
When the depression region 3p on the surface of the key top 3 is selectively depressed, the membrane switch 6s which is located opposite thereto has its movable contact 6f moved into contact with the both fixed contacts 6d and 6e, thus turning the switch on. As the key top 3 is further depressed, it moves angularly as shown in FIG. 3B and a pusher 3a extending form one side of the sidewall 3g of the key top 3 presses against a tact switch 7 which is mounted on the internal surface of a rear plate 2b of the case 2, thus turning it on.
When the key top 3 is released from the depression, a reaction which results from a resilient material within the tact switch 7 turns it off, and the restoring force of the flexible film 6b causes the movable contact 6f to move away from the fixed contacts 6d and 6e to turn the membrane switch 6s off. A flexible band-shaped cable 8 on which external connection lead wires for the contacts of the membrane switches 6s are formed by a printed circuit is taken out from the reinforcing plate 3c. 
A two-step switch which is constructed in the manner mentioned above finds its use in an application where a temporary input is selectively made initially and a true input is made after confirmation of the temporary input. However, if an on-load is relatively high when an input is to be made to the first step switch, there is a likelihood that the second step switch may be turned on inadvertently. In a portable telephone or a vehicle onboard electrical instrument, a menu is displayed on a display screen, one of items in the menu is selected by a corresponding key, and on the basis of this selection, the display screen displays what item has been selected, and a user confirms this display, and if the display is proper, the user performs a key entry in order for that item to be truly selected. In this manner, it is possible for a user to try an entry by gently depressing a suitable key (depression region 3p) without recognizing a key operation surface, which may be the display of switch identifications on the surface sheet 3e in the example of FIG. 1, to know that one color among the menu items which corresponds to the display of the switch identification for the depressed key (depression region 3p) has changed to red or that that item has been selected without requiring the visual recognition of the display of the switch identifications. If the selected item is different from an item which the user desires to select, the user may then depress another depression region 3p gently. On the contrary, if the selected item were the item which the user intended to select, an entry for that item can be accomplished by further depressing the key. In other words, a selection from the menu can properly be accomplished without viewing the key operation surface, but while viewing only the display screen. By way of example, an operation of an onboard air conditioner, a control over CD player or DVD player, a selection of a radio channel to be received, a display of TV channel to be received or a display of an automatic road guide can be made while driving an automobile.
As mentioned above, the use of a two-step switch unit is greatly convenient in making a selection or exercising a control without a visual recognition of a key display surface or while performing a different task such as driving an automobile. In this instance, it would be understood that in order to provide a distinction between the first and the second step of the two-step entry and in consideration of the fact that there is a continued need to watch a particular direction such as looking forward when driving an automobile, it is preferred that a pressure that is required to make a temporary entry through the first step switch be small in magnitude. It is desirable that the first step switch can be operated with a pressure which is as weak as “tangibly feeling” the key display surface with a fingertip or “slipping” the fingertip along the key display surface.
However, in the conventional two-step switch unit cited above, there is a need to cause an elastic deformation of the surface sheet 3e and the flexible film 6b in order to turn on the first step switch or the membrane switch 6s. This accompanies a reaction of an increased magnitude. In particular, polyethylene sheet or polycarbonate sheet is generally used for the surface sheet 3e. A relatively thick sheet is used at this end because it is disposed on the surface and its damage upon contact with an external member must be avoided. Accordingly, the sheet itself has a high reaction, and thus there has been a disadvantage that the first step switch has a relatively high on-load. As a consequence, there have been occurrences that the second step switch becomes turned on as the first step switch is attempted to be turned on, as mentioned previously. It would be greatly convenient if an operation of the first step key switch which is required to select a given display on the display screen while viewing a display condition, principally a display condition on the display screen of a portable telephone, a personal computer, a vehicle onboard instrument and the like could be achieved by tangibly feeling a key operation surface with a finger, for example, or by slipping the finger along the key operation surface. However, such has been a difficult task to achieve with a conventional two-step switch unit.
An example of a conventional depression responsive single step switch unit will be described below with reference to FIG. 23. This switch unit is disclosed in Japanese Patent No. 3,306,311 (issued Jul. 24, 2002). As a depressing piece 60 is depressed, a flexible sheet 61 becomes flexed, and a frame-shaped cushion member 62 as may be formed of urethane foam and on which the flexible sheet 61 is applied is increasingly squeezed, and a driving piece 63a of a driver 63 which is formed of a synthetic resin material and which is mounted on the internal surface of the flexible sheet 61 comes into contact with a click plate 64. When a load applied to the click plate 64 exceeds a given value, there occurs a reversal in the central portion of the click plate 64 as shown in FIG. 23B, whereby a membrane switch 6s is depressed to turn the switch on.
When the depressing piece is released from the depression, the flexible sheet 61 and the cushion member 62 which have undergone an elastic deformation return to their original configurations due to their respective resilience, and the click plate 64 also returns to its original configuration due to its resilient restoring force, whereby the switch assumes a turn-off condition. It is to be noted that a baseplate 65 is mounted on the surface of the frame-shaped cushion member 62 which is opposite from the flexible sheet 61 with the interposition of a sheet which defines the membrane switch 6s. In other words, the membrane switch 6s and the click plate 64 are secured to the baseplate 6s within an extent defined by the frame-shaped cushion member 62.
In the conventional depression responsive switch unit mentioned above, because the flexible sheet 61 on which the depressing piece 60 is mounted is secured to the cushion member 62, a drive to the click plate 64 may not take place in a satisfactory manner if a depressing force applied to the depressing piece 60 deviates from a direction which is perpendicular to the flexible sheet 61. Alternatively, if a depression is applied to one end of the depressing piece 60, the cushion member 62 will be strongly compressed toward the depressed end while it will be expanded toward the other end, causing the driver 63 to assume a relatively largely tilted position relative to the baseplate 65, preventing a drive from being transmitted satisfactorily to the click plate 64. In either instance, a load which is required to produce a reversal of the click plate 64 becomes higher than for a normal depression. This leads to problems that a clicking sensation is degraded, that a reversal may be prevented from occurring or that the useful life of the click plate 64 may be shortened.
Generally, a switch having a lower peak of on-load has a long useful life because the stresses to which the switch is subject in order to provide the clicking sensation and because the stresses to which the switch is subject from a return spring during the reversal are both small. However, if the reversal occurs as a result of a high load applied to the return spring which would occur during an edgewise depression, the return spring will be subject to correspondingly higher stresses, thus shortening the useful life.
Another example of conventional single step depression responsive switch unit will be described with reference to FIGS. 24 and 25. A depressing piece 72 faces externally through an opening 71b formed in a surface plate 71a of a case 71. When the depressing piece 72 is depressed into the case 71, a rib 72a formed on the peripheral surface of the depressing piece 72 is guided by a guide groove 71d of a tubular guide 71c which is integrally formed inside the case 71, thus moving toward a rear plate 71e of the case 71 in a direction perpendicular thereto. As a result of such movement, an actuator 73f of a tact switch 73 which is mounted centrally on the internal surface of the rear plate 71e is driven into a switch case 73b by a projection 72c which is formed centrally on the internal surface of a top plate of the depressing piece 72, whereupon an internal spring is reversed to turn the tact switch 73 on. When the depressing piece 72 is released from the depression, the original configuration is restored due to the resilient restoring force of the spring within the tact switch 73, and the depressing piece 72 is returned to its original position. It is to be noted that the rear plate 71e of the case 71 is detachable, and a screw 74 is passed through a bore 71f formed in the rear plate 71e and is screwed into a bore 71g formed in the end face of a sidewall 71i of the case 71, whereby the rear plate 71e is secured to the sidewall 71i. 
With this conventional depression responsive switch unit, the depressing piece 71 moves in a direction perpendicular to the rear plate 71e if the depression is directed obliquely and if the depression is applied to one end of the depressing piece 71. However, a friction acting between the rib 72 and the guide groove 71d increases, and it becomes necessary to increase the depressing force. A switch operation may be prohibited for a depressing force of an equal magnitude. A problem relating to the sensation of operation remains in a similar manner as in the arrangement of FIG. 23. In addition, the arrangement may become larger in size because of a guide construction for the depressing piece 72.
It is an object of the present invention to provide a depression responsive switch unit which is capable of minimizing an on-load for a plurality of first step switches.
It is another object of the present invention to provide a depression responsive switch unit which is hardly influenced by a deviation in the direction of depression or a biased depression.